Thrombin is a multifunctional protease that regulates several key biological processes. For example, thrombin is among the most potent of the known platelet activators. In addition, thrombin is essential for the cleavage of fibrinogen to fibrin to initiate clot formation. These two elements are involved in normal hemostasis but in atherosclerotic arteries can initiate the formation of a thrombus, a major factor in pathogenesis of vasoocclusive conditions such as myocardial infarction, unstable angina, nonhemorrhagic stroke and reocclusion of coronary arteries after angioplasty or thrombolytic therapy. Thrombin is also a potent inducer of smooth cell proliferation and may therefore be involved in a variety of proliferative responses such as restenosis after angioplasty and graft-induced atherosclerosis. In addition, thrombin is chemotactic for leukocytes and may therefore play a role in inflammation. (Hoover, R. J., et al. Cell (1978) 14:423; Etingin, O. R., et al., Cell (1990) 61:657.) These observations indicate that inhibition of thrombin formation or inhibition of thrombin itself may be effective in preventing or treating thrombosis, limiting restenosis and controlling inflammation.
The formation of thrombin is the result of the proteolytic cleavage of its precursor prothrombin at the Arg-Thr linkage at positions 271-272 and the Arg-Ile linkage at positions 320-321. This activation is catalyzed by the prothrombinase complex, which is assembled on the membrane surfaces of platelets, monocytes, and endothelial cells. The complex consists of Factor Xa (a serine protease), Factor Va (a cofactor), calcium ions and the acidic phospholipid surface. Factor Xa is the activated form of its precursor, Factor X, which is secreted by the liver as a 58 kd precursor and is converted to the active form, Factor Xa, in both the extrinsic and intrinsic blood coagulation pathways. It is known that the circulating levels of Factor X, and of the precursor of Factor Va, Factor V, are on the order of 10.sup.-7 M. There has been no determination of the levels of the corresponding active Factors Va and Xa.
The complete amino acid sequences of human Factor X and Factor Xa are known. FIG. 1 shows the complete sequence of the precursor form of Factor X as described by Davie, E. W., in Hemostasis and Thrombosis, Second Edition, R. W. Coleman et al. eds. (1987) p. 250. Factor X is a member of the calcium ion binding, gamma carboxyglutamyl (Gla)-containing, vitamin K dependent, blood coagulation glycoprotein family, which also includes Factors VII and IX, prothrombin, protein C and protein S (Furie, B., et al., Cell (1988) 53:505).
As shown in FIG. 1, the mature Factor X protein is preceded by a 40-residue pre-pro leader sequence which is removed during intracellular processing and secretion. The mature Factor X precursor of Factor Xa is then cleaved to the two-chain form by deletion of the three amino acids RKR shown between the light chain C-terminus and activation peptide/heavy chain N-terminus. Finally, the two chain Factor X is converted to Factor Xa by deletion of the "activation peptide" sequence shown at the upper right-hand portion of the figure (numbered 1-52 ), generating a light chain shown as residues 1-139, and a heavy chain shown as residues 1-254. These are linked through a single disulfide bond between position 128 of the light chain and position 108 of the heavy chain. As further indicated in the figure, the light chain contains the Gla domain and a growth factor domain; the protease activity resides in the heavy chain and involves the histidine at position 42, the aspartic acid at position 88, and a serine at position 185, circled in the figure.
There are two known pathways for the activation of the two-chain Factor X in vivo. Activation must occur before the protease is incorporated into the prothrombinase complex (Steinberg, M., et al., in Hemostasis and Thrombosis, Coleman, R. W., et al. eds. (1987) J. B. Lippencott, Philadelphia, Pa., p. 112). In the intrinsic pathway, Factor X is cleaved to release the 52-amino acid activation peptide by the "tenase" complex which consists of Factor IXa, Factor VIII and calcium ions assembled on cell surfaces. In the extrinsic pathway, the cleavage is catalyzed by Factor VIIa which is bound to a tissue factor on membranes. Of interest herein, however, is the ability to convert Factor X to Factor Xa by in vitro cleavage using a protease such as that contained in Russell's viper venom. This protease is described by DiScipio, R. G., et al., Biochemistry (1977) 6:5253.
Returning to the function of Factor Xa per se, the activity of Factor Xa in effecting the conversion of prothrombin to thrombin is dependent on its inclusion in the prothrombinase complex. The formation of the prothrombinase complex (which is 278,000 fold faster in effecting the conversion of prothrombin to thrombin than Factor Xa in soluble form) has been studied (Nesheim, H. E., et al., J Biol Chem (1979) 254:10952). These studies have utilized the active site-specific inhibitor, dansyl glutamyl glycyl arginyl (DEGR) chloromethyl ketone, which covalently attaches a fluorescent reporter group into Factor Xa. Factor Xa treated with this inhibitor lacks protease activity, but is incorporated into the prothrombinase complex with an identical stoichiometry to that of Factor Xa and has a dissociation constant of 2.7.times.10.sup.-6 M (Nesheim, M. E., J Biol Chem (1981) 256:6537-6540; Skogen, W. F., et al., J Biol Chem (1984) 256:2306-2310; Krishnaswamy, S., et al., J Biol Chem (1988) 263:3823-3824; Husten, E. J., et al., J Biol Chem (1987) 262:12953-12961).
Known methods to inhibit the formation of the prothrombinase complex include treatment with heparin and heparinlike compounds. This results in inhibition of the formation of the complex by antithrombin III in association with the heparin. Other novel forms of Factor Xa inhibition include lipoprotein-associated coagulation inhibitor (LACI) (Girard, T. J., et al., Nature (1989) 338:518; Girard, T. J., et al., Science (1990) 248:1421), leech-derived antistatin (Donwiddie, C., et al., J Biol Chem (1989) 264:16694), and tick-derived TAP (Waxman, L., et al., Science (1990) 248:593). Alternatively, agents which inhibit the vitamin K-dependent Gla conversion enzyme, such as coumarin, have been used. None of these approaches have proved satisfactory due to lack of specificity, the large dosage required, toxic side effects, and the long delay in effectiveness.
Accordingly, the invention offers an alternative approach of enhanced specificity and longer duration of action in inhibiting the formation of an active prothrombinase complex.